High-efficiency Conversion Behavior Of Traditional Desilication Products To Andradite

With the rapid decrease of bauxite grade and the increase of red mud stock in China,the development and innovation of efficient and economic treatment technology of low-grade aluminum resources is the fundamental to ensure the healthy,rapid and sustainable development of alumina industry in China.The preliminary study shows that the andradite(3CaO·Fe2O3·mSiO2·nH2O)is an efficient desilication product,which can greatly improve the alumina recovery rate of the wet treatment of low-grade aluminum resources and make the dissolved red mud basically free of alkali.Based on the thermodynamic analysis of the conversion of the traditional DSPs to the andradite,this paper studies the mineral phase evolution and microstructure characteristics of alkali thermal transformation from sodium aluminosilicate hydrate and grossular hydrogarnet to the andradite under different conditions by adding lime and sodium ferrite.The main conclusions are showed as follows:Based on the calculation of the thermodynamics of the formation of andradite,the thermodynamic behaviors of the transformation of sodium aluminosilicate hydrate and grossular hydrogarnet into andradite were studied.The results show that the sodium aluminosilicate hydrate in the form of zeolite and sodalite can be converted to andradite,but the possibility to grossular hydrogarnet first and then to andradite is greater;grossular hydrogarnet with different molecular formula can be converted to andradite.The effects of solution concentration and reaction conditions on the conversion of sodium aluminosilicate hydrate to andradite were studied systematically under different amounts of sodium ferrite and lime,and the optimal conditions were determined.With the increase of the amount of sodium ferrite,the extraction rate of alumina increases gradually;the addition of a proper amount of lime is helpful to recover the alkali and aluminum,but when the amount of lime is too much,the extraction rate of alumina will decrease;the higher the molecular ratio of sodium aluminate solution,the higher the extraction rate of alumina;when the reaction temperature is lower than 180?,the conversion of sodium aluminosilicate hydrate is very difficult,When the temperature is higher than 200?,the extraction rate of alumina increases with the increase of temperature.The conditions of alkali thermal transfoamation from sodium aluminosilicate hydrate to andradite are showed as follows:F/A=1.0,C/S=2.5,NK=220g/L,?K=20.4,T=260?,t=1h,L/S=5,Under this condition,the extraction rate of alumina is about 60%,and the content of Na2O in solid phase is only 0.24%.When the temperature is higher than 200?,sodium aluminosilicate hydrate will first dissolve in the alkali thermal reaction process,and a large number of grossular hydrogarnet will be generated.In the formation process of grossular hydrogarnet,Al2O3 will be replaced by the dissolved Fe2O3 and transformed into andradite,and the higher molecular ratio of the solution and temperature,the more andradite will be generated;as the reaction continues,the large particle grossular hydrogarnet will continue to transform to andradite,and with the extension of reaction time,the replacement rate of Fe2O3 will gradually increase until the replacement of Fe2O3 and Al2O3 reaches equilibrium.The effects of solution concentration and reaction conditions on the conversion of grossular hydrogarnet to andradite and the reaction mechanism were studied.When the reaction temperature is lower than 220?,it is difficult to transform the grossular hydrogarnet.With the increase of temperature,the extraction rate of alumina increases gradually;the higher the molecular ratio of sodium aluminate solution,the higher the extraction rate of alumina.At the condition of F/A=1.0,NK=220g/L,?K=20,t=lh,T=300?,L/S=5,the extraction rate of alumina was 52.37%.There are two ways in the process of transformation from grossular hydrogarnet to andradite:on the one hand,the grossular hydrogarnet gradually dissolves in the process of alkali thermal transformation,the dissolved ions recombine in the solution,directly form small particles of andradite and gradually aggregate and grow up,and the higher the temperature is,the faster the grossular hydrogarnet dissolves,the greater amount of andradite is,and the replacement rate of Fe2O3 in the andradite gradually increases to stable state;on the other hand,the surface of grossular hydrogarnet particles is in contact with the solution,and Fe2O3 in the solution will be replaced with Al2O3 in the solid solution then form andradite.However,due to the large size of grossular hydrogarnet particles,it is difficult for the inner grossular hydrogarnet to react with the solution,resulting in the low conversion efficiency of grossular hydrogarnet to andradite.At last,the alkali thermal transformation behavior and mechanism of the grossular hydrogarnet formed under different conditions were studied.The lower the formation temperature of grossular hydrogarnet is,the higher the alumina extraction rate is in the process of alkali thermal transformation to andradite;when the formation temperature of grossular hydrogarnet is 145?,the alumina extraction rate can reach over 67%.With the increase of formation temperature of grossular hydrogarnet,its silicon saturation coefficient increases gradually,which makes the grossular hydrogarnet become more stable,thus making the conversion of grossular hydrogarnet to andradite more difficult.